As a method of forming piezoelectric resin film, commonly known is a so called co-evaporation method in which a polyurea film is formed by simultaneously evaporating a diisocyanate compound such as 4,4′-diphenylmethane diisocyanate (MDI) and a diamine compound such as 4,4′-diaminodiphenyl methane (MDA), for example (refer to Patent Document 1).
However, there is a problem such that a monomer composition ratio in the resulting polyurea film differs from a stoichiometric composition ratio because of a different evaporation temperature of each monomer, since the polyurea film is arranged to be formed by simultaneously evaporating two kind of monomers of diisocyanate compound and a diamine compound in the case of the foregoing conventional technique, and evaporation has to be done while controlling temperature after setting the temperature of each of monomers separately. Also disclosed is reaction with water after limiting a monomer to only diisocyanate in order to avoid this temperature control (refer to Patent Document 2), but there is a drawback of limiting monomer selection. On the other hand, in the case of evaporation polymerization, it takes a long time to form a thick evaporation film, and there is another problem such that a vacuum system is required to produce a large-area piezoelectric film, and as a result, investment in facilities becomes large.    (Patent Document 1) Japanese Patent O.P.I. Publication No. 7-258370    (Patent Document 2) Japanese Patent O.P.I. Publication No. 2006-063393